The Variation in Chemical Composition and Source Apportionment of PM2.5 before, during, and after COVID-19 Restrictions in Zhengzhou, China

Despite significant improvements in air quality during and after COVID-19 restrictions, haze continued to occur in Zhengzhou afterwards. This paper compares ionic compositions and sources of PM2.5 before (2019), during (2020), and after (2021) the restrictions to explore the reasons for the haze. The average concentration of PM2.5 decreased by 28.5% in 2020 and 27.9% in 2021, respectively, from 102.49 μg m−3 in 2019. The concentration of secondary inorganic aerosols (SIAs) was 51.87 μg m−3 in 2019, which decreased by 3.1% in 2020 and 12.8% in 2021. In contrast, the contributions of SIAs to PM2.5 increased from 50.61% (2019) to 68.6% (2020) and 61.2% (2021). SIAs contributed significantly to PM2.5 levels in 2020–2021. Despite a 22~62% decline in NOx levels in 2020–2021, the increased O3 caused a similar NO3− concentration (20.69~23.00 μg m−3) in 2020–2021 to that (22.93 μg m−3) in 2019, hindering PM2.5 reduction in Zhengzhou. Six PM2.5 sources, including secondary inorganic aerosols, industrial emissions, coal combustion, biomass burning, soil dust, and traffic emissions, were identified by the positive matrix factorization model in 2019–2021. Compared to 2019, the reduction in PM2.5 from the secondary aerosol source in 2020 and 2021 was small, and the contribution of secondary aerosol to PM2.5 increased by 13.32% in 2020 and 12.94% in 2021. In comparison, the primary emissions, including biomass burning, traffic, and dust, were reduced by 29.71% in 2020 and 27.7% in 2021. The results indicated that the secondary production did not significantly contribute to the PM2.5 decrease during and after the COVID-19 restrictions. Therefore, it is essential to understand the formation of secondary aerosols under high O3 and low precursor gases to mitigate air pollution in the future.


Introduction
Despite significant improvements in air quality since strict restrictions on residential and industrial activities were implemented to prevent the spread of COVID-19 [1][2][3][4][5][6], haze formed in the North China Plain (NCP).Many studies have reported that the efficient formation of secondary aerosol species, including nitrate and sulfate, induced occurrences of haze during the restrictions [7][8][9].The formation of secondary aerosol species changed significantly before, during, and after the COVID-19 restrictions, but the factors of this variation remain unclear.
Nitrate (NO 3 − ) and sulfate (SO 4 2− ) are two main species of secondary inorganic aerosols that contributed to the haze pollution [10,11].Their precursor gases (NO x and SO 2 ) are primarily produced by anthropogenic primary emissions.Due to COVID-19 restrictions, The meteorological conditions (pressure, temperature, relative humidity, wind speed, and wind direction) were measured by a weather station (WS601-UMB, Lufft Instruments, Inc., Stuttgart, Germany).The concentrations of PM2.5, PM10, O3, SO2, NOx, and CO were routinely measured through a series of pollutant detectors (5030, 5014i, 49I, 42I, 43I, 48I; Thermo Fisher Scientific, Waltham, MA, USA) at a time resolution of 5 min.A one-hour average of air pollutants and weather conditions was applied.
An online analyzer for monitoring aerosols and gases (MARGA, model ADI 2080 Applikon Analytical B. V. Corp., Petten, The Netherlands) measured the hourly mass concentrations of NH4 + , Na + , K + , Ca 2+ , Mg 2+ , SO4 2− , NO3 − , and Cl − in PM2.5 at a flow rate of 16.7 L min −1 [43,44].To ensure data accuracy, the seven-point calibration curve of each species and an internal standard solution were conducted before each campaign.In addition, sampling flow calibration and cyclone cleaning were conducted to eliminate instrumental drifts.The concentrations of organic carbon (OC) and elemental carbon (EC) in PM2.5 were measured using a thermo-optical OC/EC analyzer (Model RT-4, Sunset Lab, Tigard, OR, USA).Elements in PM2.5, including K, Ca, Zn, Cu Ni, Fe, Pb, Se, Cr, Mn, As, Co, Ti, Mo, Sc, V, Ba, Br, and Si, were quantified using an online analyzer for monitoring (AMMS-100, Focused Photonics Inc., Hangzhou, China).The formulas of major components in PM2.5 and the conversion rates of SO2 to sulfate (SOR) and NOx to nitrate (NOR) are listed in Table 1.
Table 1.Formulas and references of main components in PM2.5.
An online analyzer for monitoring aerosols and gases (MARGA, model ADI 2080 Applikon Analytical B. V. Corp., Petten, The Netherlands) measured the hourly mass concentrations of NH 4 + , Na + , K + , Ca 2+ , Mg 2+ , SO 4 2− , NO 3 − , and Cl − in PM 2.5 at a flow rate of 16.7 L min −1 [43,44].To ensure data accuracy, the seven-point calibration curve of each species and an internal standard solution were conducted before each campaign.In addition, sampling flow calibration and cyclone cleaning were conducted to eliminate instrumental drifts.The concentrations of organic carbon (OC) and elemental carbon (EC) in PM 2.5 were measured using a thermo-optical OC/EC analyzer (Model RT-4, Sunset Lab, Tigard, OR, USA).Elements in PM 2.5 , including K, Ca, Zn, Cu Ni, Fe, Pb, Se, Cr, Mn, As, Co, Ti, Mo, Sc, V, Ba, Br, and Si, were quantified using an online analyzer for monitoring (AMMS-100, Focused Photonics Inc., Hangzhou, China).The formulas of major components in PM 2.5 and the conversion rates of SO 2 to sulfate (SOR) and NO x to nitrate (NOR) are listed in Table 1.

PMF Model
The origins of PM 2.5 were analyzed using the USEPA version 5.0 positive matrix factorization (PMF) model.This model is a widely used receptor model, has high efficiency and convenience without using pollution discharge conditions, and allows additional constraints to be added into the factor profiles or contributions to reduce results' uncertainties.Input factors of the model include eight water-soluble ions (e.g., Na + , NH 4 + , K + , Mg 2+ , Ca 2+ , NO 3 − , SO 4 2− , and Cl − ) and nine target metal elements (e.g., Si, Fe, Ti, Zn, Sb, Mn, Cu, Pb, and Cr), as well as the PM 2.5 mass concentration.In this study, five source types were tested in the analysis.More details about PMF running and result diagnosis are in the Supplementary Materials.

Weather Conditions and Air Pollutants
Figure 2 illustrates the variations in wind direction (Wd), wind speed (Ws), relative humidity (RH), temperature (T), and air pollutant concentrations (PM 10 , PM 2.5 , CO, O 3 , NO 2 , NO, and SO 2 ).The average wind speed (Wd) was 1.67 ± 1.01 m s −1 during the 2020 COVID-19 restrictions, 1.79 ± 1.06 m s −1 in 2019, and 1.67 ± 1.00 m s −1 in 2021.The results indicate that the wind was weak in all periods.The average temperature (T) was 3.20 ± 4.17  and convenience without using pollution discharge conditions, and allows additional constraints to be added into the factor profiles or contributions to reduce results' uncertainties.Input factors of the model include eight water-soluble ions (e.g., Na + , NH4 + , K + , Mg 2+ , Ca 2+ , NO3 − , SO4 2-, and Cl -) and nine target metal elements (e.g., Si, Fe, Ti, Zn, Sb, Mn, Cu, Pb, and Cr), as well as the PM2.5 mass concentration.In this study, five source types were tested in the analysis.More details about PMF running and result diagnosis are in the Supplementary Materials.The average concentrations of PM10 and PM2.5 were 148.49 and 102.49µg m −3 in 2019, and they decreased by 38% and 29% in 2020 and 11% and 28% in 2021 (Figure 3), respectively, resulting in a significant decline in particles in 2020 and 2021.However, the episodic haze formed with a PM2.5 value of more than 75 µg m -3 in 2020 and 2021.At the same time, the average concentrations of NO2, NO, SO2, and CO in 2019 were 44.45 ± 25.74 µg m −3 , 24.89 ± 49.49 µg m −3 , 14.90 ± 6.11 µg m −3 , and 1.25 ± 0.49 mg m −3 , respectively.NO2, NO, SO2, and CO concentrations decreased by approximately 51%, 90%, 28%, and 6% in 2020, respectively.In addition, NO2, NO, SO2, and CO concentrations in 2021 were lower by 18%, 58%, 7%, and 11% than in 2019.NO2, NO, SO2, and CO are mainly emitted by primary emissions.The reductions in NO2, NO, SO2, and CO levels indicated that primary emissions may have decreased in 2020 and 2021.In contrast, the concentration of O3 increased from 46.70 µg m −3 in 2019 to 58.24 µg m −3 in 2020 and 56.75 µg m −3 in 2021.The The average concentrations of PM 10 and PM 2.5 were 148.49 and 102.49µg m −3 in 2019, and they decreased by 38% and 29% in 2020 and 11% and 28% in 2021 (Figure 3), respectively, resulting in a significant decline in particles in 2020 and 2021.However, the episodic haze formed with a PM 2.5 value of more than 75 µg m −3 in 2020 and 2021.At the same time, the average concentrations of NO 2 , NO, SO 2 , and CO in 2019 were 44.45 ± 25.74 µg m −3 , 24.89 ± 49.49 µg m −3 , 14.90 ± 6.11 µg m −3 , and 1.25 ± 0.49 mg m −3 , respectively.NO 2 , NO, SO 2 , and CO concentrations decreased by approximately 51%, 90%, 28%, and 6% in 2020, respectively.In addition, NO 2 , NO, SO 2 , and CO concentrations in 2021 were lower by 18%, 58%, 7%, and 11% than in 2019.NO 2 , NO, SO 2 , and CO are mainly emitted by primary emissions.The reductions in NO 2 , NO, SO 2 , and CO levels indicated that primary emissions may have decreased in 2020 and 2021.In contrast, the concentration of O 3 increased from 46.70 µg m −3 in 2019 to 58.24 µg m −3 in 2020 and 56.75 µg m −3 in 2021.The increased O 3 concentration was found in many cities (e.g., Xi'an, Shanghai, Beijing, and Hangzhou) [7,9,39].

Chemical Compositions of PM2.5
The concentration of secondary inorganic aerosols (SIAs) was 51.87 µg m −3 50.25 µg m −3 during the 2020 COVID-19 restrictions, and 45.21 µg m −3 in 2021.Co with that in 2019, the SIA concentration decreased by 3.1% in 2020 and 12.8% in contrast, the contributions of SIAs to PM2.5 increased from 50.61% in 2019 to 68.6% and 61.2% in 2021.Between 2019 and 2021, SIAs were the major components of PM a greater impact in 2020 and 2021.Compared with that in 2019, the concentration decreased by 29% during the 2020 COVID-19 restrictions and 28% in 2021, but concentration decreased by 3% and 13%, respectively.The different declines b PM2.5 and their SIAs may be attributed to NO3 − and SO4 2− formation.
The average concentrations of NO3 − , NH4 + , and SO4 2− were 22.93 µg m −3 , 13.69 and 15.25 µg m −3 in 2019 (Figure 4a).The average NO3 − and NH4 + concentratio reduced by 9.8% and 5%, respectively, during the 2020 restriction period.In contra increased by 8.6% in 2020.These results indicate that the reduction in precurso (NOx and SO2) did not result in proportionate reductions in NO3 − and SO4 2− du 2020 restriction period compared to 2019.Mismatched decreases in NO3 − leve marked NOx reductions and the puzzling SO4 2− increase during the 2020 restriction offset the decrease in PM2.5 levels in Zhengzhou.The average concentrations of 2021 were similar to that in 2019.However, the average NH4 + and SO4 2− concen decreased by 16.9% and 28.9%, respectively, in 2021.The large reductions (18% a in NO2 and NO also could not help alleviate NO3 − formation in 2021, which was re the high conversion rate of NOx to nitrate (NOR).The restriction resulted in a dec PM2.5 levels; however, air quality remained episodically bad.− levels with marked NO x reductions and the puzzling SO 4 2− increase during the 2020 restriction period offset the decrease in PM 2.5 levels in Zhengzhou.The average concentrations of NO 3 − in 2021 were similar to that in 2019.However, the average NH 4 + and SO 4 2− concentrations decreased by 16.9% and 28.9%, respectively, in 2021.The large reductions (18% and 58%) in NO 2 and NO also could not help alleviate NO 3 − formation in 2021, which was related to the high conversion rate of NO x to nitrate (NOR).The restriction resulted in a decrease in PM 2.5 levels; however, air quality remained episodically bad.
The average concentrations of OC and EC were 12.71 and 3.64 µg m −3 in 2019, accounting for 12% and 4% of PM 2.5 mass (Figure 4b).The average concentrations of OC and EC were 8.97 and 2.76 µg m −3 in 2020, 29% and 24% lower than those in 2019, respectively.In 2021, the average concentrations of OC and EC decreased by 38% and 15%, respectively.Although the average concentrations of OC and EC showed significant reductions in 2020 and 2021, the contributions to PM 2.5 were similar.The value of OC/EC was 4.31 (1.83~11.93) in 2019, 3.64 (1.99~9.71) in 2020, and 2.77 (1.16~8.74) in 2021.OC and EC are majorly emitted from fossil fuel combustion and biomass burning.Carbonaceous aerosols from biomass-burning sources have richer OC than EC, implying high OC/EC values.When OC/EC ratios were lower than 2.0, carbonaceous aerosols were from vehicular and industrial emissions.In addition, high OC/EC ratios (>2.0) indicate the presence of secondary organic aerosols (SOAs) [7,37,43].OC/EC ratios mostly exceeded 2 in 2019 and 2021 (Figure S1), suggesting carbonaceous aerosols from biomass-burning sources.The mass concentration of SOA decreased from 10.31 µg m −3 in 2019 to 5.83 µg m −3 in 2020 and 6.88 µg m −3 in 2021 (Figure 4 and Table 1).The reductions in SOA were 43.49% in 2020 and 33.21% in 2021.The changes in secondary inorganic species were much smaller than those in secondary organic aerosols.The average concentrations of OC and EC were 12.71 and 3.64 µg m −3 in 2019, accounting for 12% and 4% of PM2.5 mass (Figure 4b).The average concentrations of OC and EC were 8.97 and 2.76 µg m −3 in 2020, 29% and 24% lower than those in 2019, respectively.In 2021, the average concentrations of OC and EC decreased by 38% and 15%, respectively.Although the average concentrations of OC and EC showed significant reductions in 2020 and 2021, the contributions to PM2.5 were similar.The value of OC/EC was 4.31 (1.83~11.93) in 2019, 3.64 (1.99~9.71) in 2020, and 2.77 (1.16~8.74) in 2021.OC and EC are majorly emitted from fossil fuel combustion and biomass burning.Carbonaceous aerosols from biomass-burning sources have richer OC than EC, implying high OC/EC values.When OC/EC ratios were lower than 2.0, carbonaceous aerosols were from vehicular and industrial emissions.In addition, high OC/EC ratios (>2.0) indicate the presence of secondary organic aerosols (SOAs) [7,37,43].OC/EC ratios mostly exceeded 2 in 2019 and 2021 (Figure S1), suggesting carbonaceous aerosols from biomass-burning sources.The mass concentration of SOA decreased from 10.31 µg m −3 in 2019 to 5.83 µg m −3 in 2020 and 6.88 µg m −3 in 2021 (Figure 4 and Table 1).The reductions in SOA were 43.49% in 2020 and 33.21% , respectively, which were 40%, 31%, and 26% higher than those in 2020 and 93%, −2%, and 7% lower than those in 2021.

Nitrate and Sulfate Formations over the Three Years
It is a puzzling phenomenon that SIA contributions to PM 2.5 were higher in 2020 (~68%) and in 2021 (~60%) than in 2019 (51%), when PM − and PM 2.5 (Table S2, p < 0.01).At a given precursor concentration, higher conversion ratios led to higher NO 3 − and SO 4 2− concentrations, implying that secondary conversion ratios were important for the formation of NO 3 − and SO 4 2− .It was found that NO 3 − and SO 4 2− levels were low in the region with high precursor gases region and low conversion ratios.The results suggested that the absolute concentrations of NO 3 − and SO 4 2− were more closely related to the secondary conversion rates than the precursor concentrations.A similar phenomenon was found in the summer of 2019 and 2020 [3].

EER REVIEW
7 of 15 SO4 2-.It was found that NO3 − and SO4 2− levels were low in the region with high precursor gases region and low conversion ratios.The results suggested that the absolute concentrations of NO3 − and SO4 2-were more closely related to the secondary conversion rates than the precursor concentrations.A similar phenomenon was found in the summer of 2019 and 2020 [3].There were some exceptions in the same NOx region when NO3 − concentrations were higher in 2020 and 2021 than in 2019.In 2020 and 2021, the higher NO3 − levels in the same NOx region were closely related to the higher NOR.Meanwhile, in 2020, higher SO4 2-concentrations in the same SO2 region significantly correlated with higher SOR levels.It is equally inconceivable that higher conversion ratios occurred in 2020 and 2021 to cause high secondary formation, especially NO3 − .RH and O3 are key factors regulating the oxidation pathways for nitrate and sulfate formation [14,15,37,45].Both NOR and SOR were positively correlated with RH, as shown in Figure 6 and Table S4 (p < 0.01), suggesting that high RH favors the conversion of gaseous precursors (e.g., NOx and SO2) to NO3 − and SO4 2-.The NOR concentration was high in a high-O3 region at a given RH, while the SOR was not (Figure 6).NOR was positively correlated with O3 and O3/Ox in 2019-2021 (p < 0.01,Table S4), but SOR was not.The results indicated that NOx was efficiently oxidized by O3.O3 increased in 2020 and 2021; higher NOR values were accompanied by higher O3 and O3/Ox values in a given RH, compared with 2019 (Figure 6), whereas the conversion rate of SO2 to sulfate was not.O3 was likely There were some exceptions in the same NO x region when NO 3 − concentrations were higher in 2020 and 2021 than in 2019.In 2020 and 2021, the higher NO 3 − levels in the same NO x region were closely related to the higher NOR.Meanwhile, in 2020, higher SO 4 2− concentrations in the same SO 2 region significantly correlated with higher SOR levels.It is equally inconceivable that higher conversion ratios occurred in 2020 and 2021 to cause high secondary formation, especially NO 3 − .RH and O 3 are key factors regulating the oxidation pathways for nitrate and sulfate formation [14,15,37,45].Both NOR and SOR were positively correlated with RH, as shown in Figure 6 and Table S4 (p < 0.01), suggesting that high RH favors the conversion of gaseous precursors (e.g., NO x and SO 2 ) to NO 3 − and SO 4 2− .The NOR concentration was high in a high-O 3 region at a given RH, while the SOR was not (Figure 6).NOR was positively correlated with O 3 and O 3 /O x in 2019-2021 (p < 0.01,Table S4), but SOR was not.The results indicated that NO x was efficiently oxidized by O 3 .O 3 increased in 2020 and 2021; higher NOR values were accompanied by higher O 3 and O 3 /O x values in a given RH, compared with 2019 (Figure 6), whereas the conversion rate of SO 2 to sulfate was not.O 3 was likely the major oxidant for the high level of NO 3 − formation.

Sources during the Three Periods
The profiles of six factors were identified using the EPA PMF v5.0 in Figure 7. Factor 1 was characterized by the highly explained variations in Ni, Zn, Se, Mg 2+ , Fe, and Mn in the three periods, identified as traffic emissions.Zn is widely used as an additive for lubricants in engines, vehicular tailpipe exhausts [45], debris from brake wear, and worn tires [46,47].Factor 2 was dominated by high K + , Ba, and Mg 2+ concentrations, generally indicators of biomass burning [48,49].Factor 3 was characterized by highly explained variations in Ni, As, Cu, Ni, Mo, OC, and EC in 2019 and 2020, whereas Ba, As, OC, and EC in 2021 were identified as industrial emissions.This result was consistent with different industrial processes in the three areas.Studies have shown that steel sintering can emit large amounts of As, Zn, Ni, and Cu [21,50].Factor 4 was dominated by high Cl − , OC, and EC concentrations, generally indicators of coal combustion [21,40,51].Factor 5 included large amounts of Mg 2+ , Ca 2+ , and Si [52][53][54].Previous studies have shown that soil is also represented by a high loading of Ca, Mg, Si, and Al [47,49,54]; therefore, this factor was attributed to dust.Factor 6 was characterized by high NO3 − , SO4 2− , NH4 + , and OC content, as well as relatively high concentrations of OC.These species are mainly associated with secondary processes [46,50].Thus, this factor was classified as a secondary aerosol.

Sources during the Three Periods
The profiles of six factors were identified using the EPA PMF v5.0 in Figure 7. Factor 1 was characterized by the highly explained variations in Ni, Zn, Se, Mg 2+ , Fe, and Mn in the three periods, identified as traffic emissions.Zn is widely used as an additive for lubricants in engines, vehicular tailpipe exhausts [45], debris from brake wear, and worn tires [46,47].Factor 2 was dominated by high K + , Ba, and Mg 2+ concentrations, generally indicators of biomass burning [48,49].Factor 3 was characterized by highly explained variations in Ni, As, Cu, Ni, Mo, OC, and EC in 2019 and 2020, whereas Ba, As, OC, and EC in 2021 were identified as industrial emissions.This result was consistent with different industrial processes in the three areas.Studies have shown that steel sintering can emit large amounts of As, Zn, Ni, and Cu [21,50].Factor 4 was dominated by high Cl − , OC, and EC concentrations, generally indicators of coal combustion [21,40,51].Factor 5 included large amounts of Mg 2+ , Ca 2+ , and Si [52][53][54].Previous studies have shown that soil is also represented by a high loading of Ca, Mg, Si, and Al [47,49,54]; therefore, this factor was attributed to dust.Factor 6 was characterized by high NO 3 − , SO 4 2− , NH 4 + , and OC content, as well as relatively high concentrations of OC.These species are mainly associated with secondary processes [46,50].Thus, this factor was classified as a secondary aerosol.Secondary processes (43.09 µg m −3 ), coal combustion (29.18 µg m −3 ), biomass burning (7.25 µg m −3 ), traffic emissions (6.59 µg m −3 ), industrial emissions (3.48 µg m −3 ), and dust (1.87 µg m −3 ) were the six main sources of PM 2.5 in Zhengzhou in 2019, contributing 42.04%, 28.47%, 7.08%, 6.43%, 3.39%, and 1.82% of the PM 2.5 , respectively (Table S5).Compared with 2019, the PM 2.5 mass concentration from the secondary aerosols in 2020 was similar to that in 2019, but the contribution of this source to PM 2.5 increased by 13.32%, which was consistent with the secondary inorganic components.In 2020, the PM 2.5 mass concentrations emitted by coal combustion, biomass burning, traffic emissions, industrial emissions, and dust showed obvious decreases.However, the contribution of coal combustion to PM 2.5 mass concentrations slightly increased by 2.87%, and the contributions of other sources to PM 2.5 mass concentrations slightly decreased by 0.41~3.27%.These results indicate that the changes in primary emissions from coal combustion, biomass burning, traffic emissions, industrial emissions, and dust were similar to those in the PM 2.5 level in 2020.The PM 2.5 mass concentrations from secondary aerosols, coal combustion, and traffic emissions in 2021 were lower than those in 2019, but the PM 2.5 mass concentrations emitted by industrial emissions and dust were higher.In 2021, the contribution of secondary aerosols to PM 2.5 levels was higher by 12.92% than that in 2019.However, coal combustion's contribution declined by 12.69%, suggesting that primary emissions from coal combustion were significantly reduced in 2021, while the influence of the secondary process on the PM 2.5 decreases was small.Secondary aerosols were the main source of PM 2.5 between 2019 and 2021, playing the more dominant role in PM 2.5 levels in 2020 and 2021 despite substantial reductions in gas emissions in 2020 and 2021.The primary emissions (biomass burning, traffic, and dust) were reduced by 29.71% in 2020 and 27.7% in 2021 compared to 2019.However, the PM 2.5 concentrations from the secondary processes in 2020 and 2021 were slightly decreased, and the contribution of the secondary processes to PM 2.5 increased by 13.32% in 2020 and 12.94% in 2021.The reductions in PM 2.5 levels in 2020 and 2021 were mainly due to a decline in primary emissions rather than the secondary process.A similar scenario, with higher contributions from secondary pollution to unexpected PM 2.5 levels, was found in many cities with COVID-19 restrictions [28,38,[51][52][53][54].

Discussion
Significant improvements in air quality occurred during and after the COVID-19 outbreak in China, but haze still formed [1,2,13,34,41,55].Many studies have found that enhancing secondary pollution inhibits these anomalies' continuous decline in PM 2.5 levels [9,32,38,56].Changes in secondary aerosol formation under low precursor gas (NO x and SO 2 ) levels with high O 3 levels throughout this pandemic remain unclear.Due to restrictions on residential and industrial activities implemented to prevent the spread of COVID-19 in Zhengzhou, PM 2.5 concentrations showed a remarkable decline of 29% during the 2020 COVID-19 restrictions compared with 2019.In addition, after the COVID-19 restrictions were lifted, PM 2.5 concentrations in 2021 were 28% lower than in 2019 (Figure 3).Moreover, the average molar concentration of total nitrogen compounds (NO x + NO 3 − ) in the air decreased from 1.60 µmol m −3 in 2019 to 0.89 µmol m −3 in 2020 and 1.44 µmol m −3 in 2021.Meanwhile, that of sulfur compounds (SO 2 + SO 4 2− ) also slightly decreased from 0.39 µmol m −3 (2019) to 0.34 µmol m −3 (2020) and 0.33 µmol m −3 (2021), indicating that the reduction in nitrogen compounds during the 2020 COVID-19 restrictions was larger than that in 2021, while the decline in sulfur compounds was small.Ma et al. (2022) found that the average molar concentration of total nitrogen compounds before COVID-19 restrictions (January 1 to 23, 2020) was 2.74 µmol m −3 , obviously higher than that during the COVID-19 restrictions; meanwhile, that of sulfur compounds (0.36 µmol m −3 ) was slightly higher than that during the COVID-19 restrictions.
Despite significant decreases in total nitrogen in 2020 and 2021, NO 3 − levels were similar to that in 2019, while NO and NO 2 levels were significantly lower than that in 2019.This result indicates that nitrogen levels significantly changed when partitioning into NO 3 − , NO 2 , and NO in 2020 and 2021.It is a puzzle why a higher distribution of NO 3 − occurred with lower NO and NO 2 in 2020 and 2021 than in 2019.It was found that the O 3 concentration was 1.21 µmol m −3 in 2020 and 1.18 µmol m −3 in 2021, higher than 0.97 µmol m −3 in 2019.O 3 molar concentrations in 2020 and 2021 were far greater than NO 2 molar concentrations (0.47 µmol m −3 and 0.80 µmol m −3 ), respectively, implying that high O 3 promoted the conversion of NO to NO 2 , then to NO 3 , and finally to NO 3 − .Thus, NO levels in 2020 and 2021 were obviously lower than in 2019 (Table S1).The high production of NO 3 − from NO x could have offset the effect of reduced total nitrogen on PM 2.5 levels [1,2].
NOR positively correlated with RH in 2019-2021 (Figure 6a,b and Table S4, p < 0.01).NOR was much higher in 2020 and 2021 than in 2019, when RH was higher than 40%.This result is consistent with high RH favoring NO 3 − formation via NO x in 2020 and 2021 (Figure 6, Table S4).The increase in NOR with RH in the daytime was significantly higher than at nighttime (Figure 8).Moreover, the variations in NOR, O 3 /O x , and O 3 concentration were similar in the daytime (high) and nighttime (low) in 2019-2021 (Figure S4).Additionally, high O 3 /O x and O 3 concentrations in the daytime in 2020-2021 were accompanied by high NOR.In 2020 and 2021, high NOR meant high NO 3 − formation from NO x .As shown in Figure 6, the high NOR was positively related to high O 3 and O 3 /O x (Table S4).The formation was likely because high O 3 promoted the conversion of NO to NO 2 , resulting in more NO 2 to form NO ure 6, Table S4).The increase in NOR with RH in the daytime was significantly higher than at nighttime (Figure 8).Moreover, the variations in NOR, O3/Ox, and O3 concentration were similar in the daytime (high) and nighttime (low) in 2019-2021 (Figure S4).Additionally, high O3/Ox and O3 concentrations in the daytime in 2020-2021 were accompanied by high NOR.In 2020 and 2021, high NOR meant high NO3 -formation from NOx.As shown in Figure 6, the high NOR was positively related to high O3 and O3/Ox (Table S4).The formation was likely because high O3 promoted the conversion of NO to NO2, resulting in more NO2 to form NO3 − .If O3 concentrations in 2020 and 2021 were similar to those in 2019, the NOR might have been similar to that (0.25) in 2019.NO3 -concentrations in 2020 decreased from 20.36 µg m −3 to 11.44 µg m −3 in 2020, while NH4 + levels fell by 3.32 µg m −3 .Thus, PM2.5 levels decreased from 73.24 to 61.00 µg m −3 during the COVID-19 restriction period.Similarly, NO3 -and NH4 + declined by 5.16 µg m −3 and 1.49 µg m −3 in 2021, resulting in a decrease (~7 µg m −3 ) in PM2.5 levels.Enhanced NOR hindered a ~17% and ~10% decline in PM2.5 levels during and after the COVID-19 restrictions in Zhengzhou.As a result of the increased O3, NO3 -formation was enhanced by oxidizing NO to NO2, offsetting the reduction in PM2.5 levels.
SOR increased with RH between 2019 and 2021 (Figure 6c,d).This result indicates that moist air favors the conversion of SO2 to SO4 2-.Moreover, the variation in SOR with RH was similar in the daytime and nighttime.O3 concentration was high in the daytime and low at nighttime (Figures S4 and S5).On the contrary, SOR and RH levels were low in the daytime and high at nighttime.Figure 6a-c show that high SOR did not follow high O3 levels under certain RH values between 2019 and 2021.Figure S3 and Table S4 depict the poor correlation between SOR and O3.The results suggest that photochemical reactions were not the major pathway of the SO4 2-formation.Thus, the heterogeneous reactions on the particle surface were the major route for SO4 2-formation, rather than photochemical reactions.SOR increased with RH between 2019 and 2021 (Figure 6c,d).This result indicates that moist air favors the conversion of SO 2 to SO 4 2− .Moreover, the variation in SOR with RH was similar in the daytime and nighttime.O 3 concentration was high in the daytime and low at nighttime (Figures S4 and S5).On the contrary, SOR and RH levels were low in the daytime and high at nighttime.Figure 6a-c show that high SOR did not follow high O 3 levels under certain RH values between 2019 and 2021.Figure S3 and Table S4 depict the poor correlation between SOR and O 3 .The results suggest that photochemical reactions were not the major pathway of the SO 4 2− formation.Thus, the heterogeneous reactions on the particle surface were the major route for SO 4 2− formation, rather than photochemical reactions.
As shown in Figure S2, the slopes of NH  S6).Temperature has negative correlations with NO 3 − , SO 4 2− , and conversion ratios (NOR and SOR) (Tables S2-S4).The NO 3 − had similar levels in 2019-2021, despite lower total nitrogen compounds in 2020 and 2021 than in 2019.Thus, wind speed and temperature were not major factors for the enhanced NO 3 − .The normalized concentrations of NO 3 − by EC were to counteract the effect of the atmospheric physical processes and represent the contribution from chemical reactions [56] because EC was produced only from primary emissions and was a very inertial species to the chemical reactions.Its variability was mainly controlled by emission intensity and atmospheric physical processes.The NO 3 − /EC ratio increased from 6.30 in 2019 to 7.50 in 2020 and 7.44 in 2021, reflecting an elevated production of NO 3 − .The results were consistent with the variations in NO 3 − contributions to PM 2.5 .An earlier study also observed a similar phenomenon [38,[52][53][54].Another earlier study reported that NOR increased from 0.14 to 0.21 in 2020 when Shanghai implemented restrictions [1].In Xi'an, NOR increased from 0.33 in non-restriction periods to 0.57 in restriction periods, when O 3 concentration increased from 35.3 to 53.6 µg m −3 [38].An investigation of the published data on haze occurrence during and after COVID-19 restrictions in many cities in China revealed a similar scenario, with photochemically enhanced NOR causing major contributions of NO 3 − to PM 2.5 and inhibiting the continuous decline in PM 2.5 levels [1,9,25,38].

Conclusions
Despite significant improvements in air quality during and after COVID-19 restrictions, due to strict regulations on residential and industrial activities to curb the spread of COVID-19, haze still occurred in Zhengzhou.The average concentration of PM 2.5 decreased from 102.49 µg m −3 before the restrictions (2019) to 73.24 µg m −3 during the restrictions (2020) and 73.90 µg m −3 after the restrictions (2021).In 2019, SIA concentrations declined by 3.1% in 2020 and 12.8% in 2021.At the same time, the contributions of SIAs to PM 2.5 increased to 68.6% (2020) and 61.2% (2021), higher than 50.61% in 2019.SIAs caused major contributions to PM 2.5 levels in 2020 and 2021.NO x levels in 2020 and 2021 were 22~62% lower than that in 2019; however, NO 3 − concentrations had similar levels (20.69~23.00µg m −3 ) in 2019, 2020, and 2021, which was attributed to the enhanced conversion rate of NO x to nitrate by the increased O 3 levels in 2020 and 2021.The inconsistent reduction between nitrate and NO x inhibited further PM 2.5 reduction and contributed to haze formation in Zhengzhou.
Six of the PM 2.5 sources were secondary inorganic aerosols, industrial emissions, coal combustion, biomass burning, soil dust, and traffic emissions in 2019, 2020, and 2021.Compared with 2019, the primary emissions (biomass burning, traffic, and dust) were reduced by 29.71% in 2020 and 27.7% in 2021.However, the PM 2.5 from the secondary process in 2020 and 2021 decreased slightly, and the contribution of the secondary process to PM 2.5 increased by 12.94%~13.32% in 2020 and 2021.The reductions in PM 2.5 levels in 2020 and 2021 were mainly due to a decline in primary emissions rather than the secondary process.Thus, a better understanding of the formation of secondary aerosols under high O 3 and low precursor gases would be needed to help mitigate air pollution in the future.

Figure 1 .
Figure 1.Location of the site (Zhengzhou) and topography in China.

Figure 1 .
Figure 1.Location of the site (Zhengzhou) and topography in China.

Figure 3 .
Figure 3. Change ratios of pollutant concentrations in 2020 and 2021 compared to 2019.

Figure 3 .
Figure 3. Change ratios of pollutant concentrations in 2020 and 2021 compared to 2019.

3. 2 .
Chemical Compositions of PM 2.5 The concentration of secondary inorganic aerosols (SIAs) was 51.87 µg m −3 in 2019, 50.25 µg m −3 during the 2020 COVID-19 restrictions, and 45.21 µg m −3 in 2021.Compared with that in 2019, the SIA concentration decreased by 3.1% in 2020 and 12.8% in 2021.In contrast, the contributions of SIAs to PM 2.5 increased from 50.61% in 2019 to 68.6% in 2020 and 61.2% in 2021.Between 2019 and 2021, SIAs were the major components of PM 2.5 , with a greater impact in 2020 and 2021.Compared with that in 2019, the concentration of PM 2.5 decreased by 29% during the 2020 COVID-19 restrictions and 28% in 2021, but the SIA concentration decreased by 3% and 13%, respectively.The different declines between PM 2.5 and their SIAs may be attributed to NO 3 − and SO 4 2− formation.The average concentrations of NO 3 − , NH 4 + , and SO 4 2− were 22.93 µg m −3 , 13.69 µg m −3 , and 15.25 µg m −3 in 2019 (Figure 4a).The average NO 3 − and NH 4 + concentrations were reduced by 9.8% and 5%, respectively, during the 2020 restriction period.In contrast, SO 4 2− increased by 8.6% in 2020.These results indicate that the reduction in precursor gases (NO x and SO 2 ) did not result in proportionate reductions in NO 3 − and SO 4 2− during the 2020 restriction period compared to 2019.Mismatched decreases in NO 3

−
2.5 decreased from 102.49 µg m −3 (2019) to 73.92 µg m −3 (2020) and 76.78 µg m −3 (2021).SO 2 and NO x are NO 3 − and SO 4 2− 's key precursor gases.To elucidate the puzzling phenomenon, the formation of NO 3 − and SO 4 2− was compared with their precursor gases, PM 2.5 , and conversion ratios under different periods.NO 3 − positively increased with NO x between 2019 and 2021 (Figure 5 and Table S2, p < 0.01), indicating that high concentrations of the precursor contributed to high NO 3 Toxics 2024, 12, 81 7 of 15 levels and then led to the remarkable increases in PM 2.5 .The result was supported by the positive relationship between NO 3

Figure 6 .
Figure 6.Correlations between conversion ratios (NOR and SOR) and RH in 2019 (a,d), 2020 (b,e), and 2021 (c,f).Symbols in (a-f) are scaled by O3 concentration and colored by O3/Ox.The oxidants (Ox) are the sum of NO2 and O3, a proxy for atmospheric oxidation caused by photochemical reactions.

Figure 6 .
Figure 6.Correlations between conversion ratios (NOR and SOR) and RH in 2019 (a,d), 2020 (b,e), and 2021 (c,f).Symbols in (a-f) are scaled by O 3 concentration and colored by O 3 /O x .The oxidants (O x ) are the sum of NO 2 and O 3 , a proxy for atmospheric oxidation caused by photochemical reactions.

3 −
. If O 3 concentrations in 2020 and 2021 were similar to those in 2019, the NOR might have been similar to that (0.25) in 2019.NO 3 − concentrations in 2020 decreased from 20.36 µg m −3 to 11.44 µg m −3 in 2020, while NH 4 + levels fell by 3.32 µg m −3 .Thus, PM 2.5 levels decreased from 73.24 to 61.00 µg m −3 during the COVID-19 restriction period.Similarly, NO 3 − and NH 4 + declined by 5.16 µg m −3 and 1.49 µg m −3 in 2021, resulting in a decrease (~7 µg m −3 ) in PM 2.5 levels.Enhanced NOR hindered a ~17% and ~10% decline in PM 2.5 levels during and after the COVID-19 restrictions in Zhengzhou.As a result of the increased O 3 , NO 3 − formation was enhanced by oxidizing NO to NO 2 , offsetting the reduction in PM 2.5 levels.

Table 1 .
Formulas and references of main components in PM 2.5 .
The enhanced formation of nitrate and sulfate in 2020 and 2021 may not have been caused by rich NH 4 + .The wind speed was generally less than 2 m s −1 in 2019-2021 (Figure 4 + and NO 3 − + 2 SO 4 2− were 1.11 in 2019, 1.02 in 2020, and 1.02 in 2021, implying that NH 4 + was rich in 2019.